Democracy and idealism do not sit well together. Idealists have the motivation to form political parties and keep them going. But in order to win power the party must bring on board people and, policies, that the idealists disagree with, in order to win round those less committed to politics. And these floating voters come to matter more to the party's managers than the the idealists. Because the idealists have nowhere else to go.
In Britain, the latest challenge to this process comes from Britain's Labour Party; in America the Republicans seem to be doing something similar. This all seems to be part of the great cycle of politics. A party's core supporters, those that are ideologically committed, get fed up with being taken for granted and rebel. They struggle to accept that a majority of voters disagree with them - following a natural human bias that most people think as we do. They may also be enticed by the idea that they can win by accident - through their opponents' mistakes. Sometimes such ideological parties do win an election that way - it has just happened in Poland, for example. It rarely ends well.
I know more about the Labour phenomenon than the Republican one. Labour members elected the ideological Jeremy Corbyn after the party's general election failure last year. These members remain as fervent as ever, and indeed new members have flocked in. This burst of enthusiasm has convinced them that they have started a new and better form of politics. As they see it, the compromises used to chase the centre ground, as uncommitted voters are usually referred to, have disillusioned people with politics. Now Labour will create a sharper narrative that will go down a storm with the electorate. They equate their own disillusionment with the compromises of their party with the widespread political apathy of the population at large.
But is this is an illusion. This week Britain's polling organisations published a report into why they called the 2015 election wrongly. They overestimated Labour support and underestimated the Conservatives'. They found this was mainly because their samples were biased towards Labour. And that was because they were biased towards the politically committed, who were much easier to reach. This is a vulnerability of the quota sampling technique that the pollsters use. The less committed, or more apathetic, voters were much more likely to vote Tory.
This leaves more thoughtful Labourites with two headaches. The first is that current polls show the Labour vote holding up compared to the general election - so that electing Mr Corbyn at least hasn't made things worse. But if the polling bias remains (and it seems to be, based on how the samples remember they voted in 2015), then in fact the Tory lead has grown. The second headache is that the army of the apathetic non-voters is more sympathetic to the Tories than many suppose.
Which leads to an inevitable conclusion. In order for Labour to win an election they need to convert people who voted Conservative last time, or who did not vote, but lean to the Conservatives. In other words, Labour must appeal to the centre ground.
Such thoughts cut no ice with Labour's new members. When pushed they even suggest that winning is not that important. That leaves Labour in a terrible position, and the Conservatives thinking that they have the next election in the bag. Some hope that the European referendum will split the Tories. But the prospect of whacking Labour really hard if they hold together is the best possible incentive to hold the party together.
Labour's prospects against the SNP in Scotland are no better; the SNP have cornered the middle ground in Scotland as masterfully as the Conservatives in England, while still retaining a substantial core vote. This conjuring trick will eventually come apart - but an ideological Labour Party will not be the instrument of the SNP's demise.
Meanwhile, sitting on the sidelines are the Lib Dems. A number of people have suggested to me that Labour's woes present the party with a golden opportunity. But the political dynamics or the core and centre are not working the party's favour.
The party thought that the usual rules of politics would apply to them when they went into coalition with the Conservatives in 2010. They shafted their core voters, but surely they had nowhere else to go? And meanwhile the party's record in government would appeal to the centre ground. But a large part of what the Lib Dems thought was their core vote felt they did have an alternative: Labour. That weakened the party, and weakness is a big turn-off for centrist voters. The Conservative campaign exploited this ruthlessly, and the result was catastrophe, as the Lib Dem vote fell by two thirds, and their political clout even further.
So, somehow, the Lib Dems need to rebuild their core vote. The place to look is amongst Labour inclined voters who do not buy Labour's new sense of direction. But the party also needs to win votes back centrist voters from the Conservatives if they are to win the all-important parliamentary seats. And that means the party must show distance from the Labour Party. So how does the party face the prospect of another coalition with the Conservatives? If they rule it out, they will lose the middle ground by giving tacit support to the ideological Labour Party. If they don't, those Labour inclined "core" voters will think that the party has learned nothing from the coalition debacle, and leave the party alone.
This may not matter too much to the party at the next election, especially if it looks as if the Tories will win handsomely. There will be no danger of a coalition, so that awkward question can be ducked. The Lib Dems might be able to make a modest recovery based on local strength. But the strategic dilemma remains.
Probably the best thing for the party is to recognise that it is essentially of the left, and rule out any future coalition with the Conservatives. That will help the party rebuild its core. It then needs to apply thought to under what conditions it could work with Labour. But it will have to be a very different Labour Party from the one emerging under Mr Corbyn's leadership.
Which would leave the middle ground in British politics to the Conservatives and the SNP. Which in turn means that political power will rest with them. A grim prospect indeed.